Enriching the alumina content of recirculated cryolite fusions in aluminum production



ENRICHING THE AIJUMINA CONTENT OF RECIRCULATED CRYOLITE FUSIONS IN ALUMINUM PRODUCTION 3 Sheets-Sheet 1 Oct. 19, 1948. A JOHNSON 2,451,491

Filed Jan. 30, 1945 LUMINIFEROUS ORE IL. CALCINNG FURNACE V I A UsmcL-rme F9955: 40' x I I v I 4 I l x 47 J I n7 i v Ream-non FURNACE. I l

I i 49 FIRST CARBONACEOUS un-ea SECOND Mums I FILTER-u A i n v I? t 1 0d i f l 0\ 1 i l4- 2 [4, SIXTIETH CELL FI TY'N NTH CELL SECOND CELL FIRST CELL g3" I I INSULATION E 1 22 20- E l 27 i I 2. z Z9 2 l I 5 26' 1 Z6 22 i a i L- FT -'1': II I I i9 I u INVENTOR ARTHUR I? dO/l/VJON ATTORNEYS A. F. JOHNSON ENRICHING THE ALUMINA CONTENT OF HECIRCULATED Oct. 19, 1948.

CRYOLITE FUSIONS IN ALUMINUM PRODUCTION 5 SheetsShet 2 Filed Jan. 30, 1945 CHEEDA/ UNI/V6 ammo/v I ,1 H, H

mew/v CflEBU/V 6477/0055 mesa/v 0/1005 q INVENTQR ARTHUR E JOH/WJON ATT NEYS Oct. 19, 1948. A. F. JOHNSON 2,451,491

ENRICHING THE ALUMINA CONTENT OF REGIRCULATED CRYOLITE FUSIONS IN ALUMINUM PRODUCTION Filed Jan. 30, 1945 3 Sheets-Sheet 5 INVENTOR AR THU/7 fi JOl/NJON ATTORNEYS Patented Oct. 19, 1948 ENRICHING THE ALUMINA CONTENT OF RE- CIRCULATED CRYOLITE FUSIONS IN ALU- MINUM PRODUCTION Arthur F. Johnson, Cambridge, Mass, assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Application January 30, 1945, Serial No. 575,313

4 Claims.

My invention relates to improvements in the production of aluminum, the metal, from ores containing hydrated aluminum oxides, commonly designated bauxites. Such ore-s usually co substantial proportions of combined iron, silicon and titanium, but the presence of substantial amounts of these elements in aluminum produced from such ores seriously impairs the quality of the metal.

For many years the aluminum of the World has been produced by electrolytic reduction of the Hall process of alumina prepared from bauxite, generally by the Bayer process. The Bay-er process and the Hall process are both well detailed in the technical literature; see The Aluminum Industry, by Edwards, Frary and Jeffreys, Aluminum and Its Production, (McGraw-Hill, 1930), pages 124-131 for the Bayer process and pages 300-318 for theHall process. My invention is an improvement of this long-established conventional practice. In one aspect, my invention is an improvement of the process described in my application filed August 4, 1944, Serial No. 548,043.

My invention provides improvements in the direct production of fluoride fusions containing dissolved alumina, the electrolyte of the Hall process, of high purity from ores which may contain substantial proportions of impurities such as combined iron, silicon and titanium and improvements in the combined operation including the electrolytic reduction. My invention thus provides for the production of metalof high quality with improved economy and simplified operation.

My invention involves a novel double filtration through carbonaceous material of the fluoride fusion, first following a step in which alumina is dissolved from the aluminiferous material in the fusion in the presence of carbonand second following a step in which aluminum is reacted with the once filtered fusion. Thus, the complete process of my invention comprises five steps: (1) solution of alumina in the fluoride fusion in the presence of carbon as in a, smelting operation to which the ore and the fusion are supplied, (2) filtration of the fusion through carbonaceous mate'- rial such as a body of coke, (3)- reaction of the fusion filtered in the second step with aluminum, (4) filtration of the fusion reacted with aluminum through carbonaceous material and (5) electrolytic reduction of the twice filtered fusion. The electrolysis is with special advantage carried out in a series of cells through which the fusion is circulated from the second filtration to the solution or smelting operation in repetitions of this cycle. In step 1, the bulk of the silicon and of the iron and titanium are removed, being separated from the fusion as an alloy which is recovered as a by-product. For example, or more of the iron may thus be eliminated from the fusion. In step 2, the bulk of the remaining silicon, iron and titanium are removed on the carbonaceous material. In step 3, the bulk of the remaining traces of silicon, iron and titanium are reduced and thus rendered insoluble in the fusion, probably to elemental form but perhaps to some extent to carbides. The reduced and insoluble material appears in the fusion in finely dispersed condition, corresponding to the state of dispersion of the impurities remaining prior to this reduction. In step 4, the reduced and insoluble material is removed on the carbonaceous material. The treatment of the fusion with aluminum in step 3 apparently serves a double function in the complete process of my invention; the aluminum is a, pcculiarly effective reducing agent and it also seems to promote the removal of the finely dispersed metal or carbides or both by promoting wetting of the carbonaceous material used as a filter by the fusion carrying the dispersed reduced and insoluble material. A fluoride fusion charged with dissolved alumina of high purity is thus produced directly from the aluminiferous material. In step 5, this fusion is subjected to electrolytic reduction to produce aluminum metal of correspondingly high quality.

In the Hall process, alumina is subjected to electrolysis in solution in fused cryolite at a temperature approximating, or perhaps somewhat less than, 1000 C. between carbon electrodes with a voltage drop across each cell of the order of 5-6 volts. Sometimes other salts, fluorides, and fluorspar in particular, are added to the cryolite to lower the melting point of the fusion or to increase the solubility of alumina in the fusion. Additions of sodium fluoride, for example, tend to increase the solubility of alumina in the fusion. In domestic practice, the fusion usually includes other fluorides, and the range of concentration of dissolved alumina approximates 1%-6% of the fusion. In foreign practice, cryolite is sometimes used without added fluorides and the upper limit of the range of alumina concentration in the fusion is higher. Direct current in high amperages being economically available at voltages of about 600-700, it is common practice to operate say such cells in series electrically.

In a typical single conventional cell: The bottom electrode, the cathode, may be 75 inches Wide and 148 inches long and the top electrode, the anode, ay be 45 inches wide and 120 inches long,

the liquid electrolyte may aggregate 8000 lbs. and the solid electrolyte, in theform of a crust cover ing the liquid electrolyte may aggregate 7000 lbs. (excluding electrolyte carried into the cell lining by electro-o-smosis). The fused metal, tapped every three days forexample, may-vary in depth over the'bottom electrode from 'a'minimum of about 1 or 2 inches to a maximum of about 6 inches, the depth of liquid electrolyte over the fused metal being about 9 inches, and the power may approximate 32,000 amperes at about 5.5 volts, the current efficiency being about 85%. As the alumina dissolved in the fusion is consumed by the electrolysis, fresh alumina'isaddedfrom time to time. In normal operation, fresh alumina is distributed over the frozen crust on the electrolyte from time to time, where itdspreheated, and as required is manually stirred ,or poked through the crust into the liquid electrolyte. In the event of irregular operation due to a deficiency of alumina in'the-electrolyte, cold-alumina mayjbe stirred into theel-ectrolyte to overcome-the so-called anode effect without any period of preheating on the-crust covering the electrolyte in the cell. A single iabcrer can usually tend eight cells'for his shift aud t-he operation of each of the cells isa separate operation, in effect a batch operation since alumina is charged and consumed and charged againwhen consumed and so on, althou'ghthe electrolysis is carried out continuously.

' Satisfactory operation requires control in a number of respects. Ifthe temperature much exceeds 11000 C.,*fiuorine and aluminum fluoride are lostyby vaporization yet'a temperature above the'melting point of the electrolyte must be maintained'in the region of electrolysis. In regular operation, temperature control is effected by adjusting the distance-between the electrodes in .each' cell-"appropriately with respect to the composition-of the electrolyte in the cell. If the concentration of dissolved alumina in the fusion becomes-too low, loss thanabout' 1 the fluorides apparently are electrolyzed, an envelope of gas forms on-and insulates the upper electrode, and overheating of the c'ell occurs. This-is the socalled fanode effect. This condition is remedied by adding-alumina to the fusion which, dissolvin-g,--lowers the resistance "of the fusion and restores normal operating conditions thus reducing excessivetemperatures. If the concentration of solid alumina inthe fusion'becomes' high enough to perm-it undissolved alumina to reach the layer of fusedmetal' covering the bottom electrode, such 'solid'alumina tends to sink into the fused metal, forming an insulating layer on the cathode and rapidly increasing the electrical resistance o-f'the cell, and as a consequence the heat liberation within and the power consumption of the cell.

There is no certain remedy for this condition other tha-ntocut the cell out of operation until 'it has cooled ofiand put it back in operation'i-n the regular way after any necessary repairs.

*Time is required for solution of alumina in the cryolite fusion and consequently the condition just described may occur even though the alumina concentration iswell withinthe solubility limit of the fusion if, for example; the alumina is added di-ssolved alumina, it would not operate satisfactorily-With a fusion containing 6% of dissolved and 4% of undissolved alumina.

V The Hall process, as conventionally practiced, requires a regular supply of fresh alumina of high purity. The impurities commonly associated with aluminum in the raw ore, iron, silicon, and titanium, being more electropositive than aluminum, contaminate the :zmetal 'ilikrerated" by electrolysis as-alloying elements tothe ezcten t -that they are present in the alumina charged to the process.

, caustic soda to form a solution of sodium aluminate supersaturated with respect to aluminum trhhyldrate, impurities insoluble in or slowly soluble in this solution, including compounds of. iron, silicon, and titanium, are separated "from this solution as =a mud,'-by-settling, filtration. or a combination of the two, aluminum tri-hydrate is-then 'precipitatedfrom this solution by seeding, and the aluminum tri-hydrate separated from .the seededsolution, .aftersett-ing aside the seed crystals required to continue the process, is calcined to producealumina of high purity. The recovered solution, .after regeneration with added caustic soda or; lime andxsodaaash, is re'-used cyclically in the digestion. 'In some plants the. sodium alu min-ate solution, instead of being formed by digestion as in. the Bayer process, is formed by sinteringthe ore with lime or lime and'soda and by extracting.thehiluminate from the sintered-prodnot with water or aqueous caustic soda, the

Deville-Pechiney process. The sodium-aluminate.

solution-sisthenxprocessedas in the Bayer :process to recover alumina. In either case, the producring out my invent-lama arrange; the several cellsof aseries, say 60- cells, ona slight gradient-Just enoughtomaintain flow of theifused electrolyte, I connectthe .upper portion of that part of:the

cell chamber normally occupied by liquid elec-,

tnolyte :of each: cell to thenext in the seriesby a trough, thermally well insulated-and with. appropriateinsulationto separate the successive cell chambers electrically, 1 provide a smelting turn'ace to wl-iich the bauxite or other aluminiferous material is charged together with carbon, as coke for xamplaandtowhich Iitra-nsfer fusion from the last cell of the series, I provide a reaction furnace-in which fusion fromthe smelting furnace is reacted with'aluminum, and I provide a pair of filters charged-with carbonaceous materiaL'on e through -:which the'fusion passes'from' the smelting furnacev to the reaction furnace and another thrcugh which'. the fusion passes "from the reaction furnace and from which I transfer the regenerated fusion to the firstcell of the series. I may provide a pair of thermally well insulated resere "voirs connected to the cell chambers :of the first 'a-nd las-tc'ells of the series and then may transfer fusi'on to an-d from these reservoirs rather than" to andffrom the-fi'rstan-d last'cells,respectively, or such reservoirs may 'lo'e omitted. Two such series of' 60 cells again connected in electrical series wi'llta'ke theplace of the conventional battery of 120 cells. The conductivity of the electrolyte being relatively low, I make the current losses through the stream of electrolyte connecting successive pairs of cells negligible by making these troughs lon with respect to the distance between the electrodes in each cell and by restricting their cross section. Transfer of electrolyte by ladle to the smelting furnace or from the second filter, for example, breaks the electrical circuit through this part of the cycle of movement of the electrolyte. For example, to connect cells of the conventional construction previously described, passage through the several troughs may be made about inches to inches long and about 6 inches wide and deep enough to provide for a stream of liquid electrolyte about 4 inches deep covered with a crust of frozen electrolyte of about the same depth. The cycle of movement of the electrolyte is also a cycle of variation of alumina content from a maximum entering the first cell to a minimum leaving the last. Since I dissolve the alumina in the fusion before the fusion is supplied to the first cell of the series, I no longer need be concerned with the rate of solution of the alumina in the fusion in the cell and, as a consequence, I can safely charge a fusion containing a high concentration of alumina to the first cell since the alumina is dissolved before the fusion enters the cell. Then, havin thus been able to raise the initial concentration, I can economically carry a higher than normal minimum concentration of alumina in the fusion leaving the last cell of the series to avoid occurrence of the anode effect. For example, the concentration of alumina in the fusion as it moves through the series of cells may vary from about 12%14% to about 2%-2.5% in each cycle. Having selected a minimum concentration, circula- .tion of the electrolyte is maintained at a rate sufficient to maintain this minimum concentration. The maximum concentration is maintained by appropriate addition of aluminiferous ore to the smelting furnace. There, and in the associated reaction furnace and the two filters, the alumina content of the ore is dissolved in the fusion and the impurities, particularly iron, silicon and titanium are removed to produce the electrolyte of high purity charged to the first cell of the series containing alumina in maximum concentration. The purification is further controlled, in accordance with my invention, by controlling the NaF:AlF3 ratio in the fusion as it passes through the smelting furnace, the reaction furnace and the two filters. If this ratio is maintained at a value exceeding 150:100, iron is removed almost quantitatively and titanium removal is satisfactory but the residual combined silicon tends to increase; if this ratio is maintained at a value less than 1501100, silicon is removed almost quantitatively and the purification with respect to iron and titanium is satisfactory. I have found a ratio of about 1451100 to be particularly advantageous. This ratio is easily adjusted by appropriate additions, to the smelting operation for example, of sodium fluoride or aluminum fluoride. The first filtration is, with advantage, carried out in a coke filled tower from which coke saturated with impurities is transferred to the smelting operation, the iron, silicon and titanium separated in the filter thus being recovered as part of the alloy separated as a byproduct in the smelting operation. The quantity of material separated from the fluoride fusion in the second filtration, although important withrespect to the quality of the metal produced by subsequent electrolysis of the fusion, is relatively small compared to that separated in the first filtration and the quantity of carbonaceous filtering material to be handled in the second filtration is correspondingly small.

I have illustrated diagrammatically and conventionally, in the accompanying drawings, one form of apparatus appropriate for carrying out my invention and I have, in Fig. 1, diagrammed the process of my invention.

In the accompanying drawings:

Fig. l is a flow diagram illustrating the practice of the process of my invention;

Fig. 2 is a vertical section of a smelting furnace;

Fig. 3 is an elevation normal to the view shown in Fig. 2;

Fig. 4 is a vertical section of a reaction furnace;

Fig. 5 is a vertical section normal to the view shown in Fig. 4;

Fig. 6 is an elevation in section of three of a series of cells arranged for practicing my invention;

Fig. 7 is a vertical section of one of the cells shown in Fig. 6 normal to the view shown in Fig. 6;

Fig. 8 is a vertical section of a transfer ladle;

Fig. 9 is a top view of the transfer ladle shown in Fig. 8; and

Fig. 10 is a bottom view of the transfer ladle shown in Fig. 8.

Referring first to Fig. 1: The several rectangles [0 represent four cells of a series of sixty. The cells are in electrical series through the several connections II, the terminal anode being indicated at l2 and the terminal cathode at l3. The several cells are also connected in chemical series by troughs I4. The cells and troughs may be constructed as illustrated in Figs. 6 and '7. A smelting furnace, such as is illustrated in Figs. 2 and 3, is represented at IT. A calcining furnace, such as a conventional rotary kiln, is illustrated at 46. A carbonaceous filter for fusion discharged from the smelting furnace is represented at 41. A reaction furnace, constructed, for example, as illustrated in Figs. 4 and 5, for treating the fusion discharged from the filter 41 is represented at 48. The rectangle 49 represents a second and smaller carbonaceous filter. Transfers of fusion between the various parts of the apparatus are conveniently effected by a ladle such as that illustrated in Figs. 8, 9 and 10.

In carrying out the process of my invention in the apparatus diagrammed in Fig. 1: The raw aluminiferous ore is charged to the calcining furnace 46 and the calcined ore, water having been substantially eliminated by the calcination, is charged to the smelting furnace l1. Coke sufficient to effect reduction of the iron, silicon and titanium present in the ore is also charged to the smelting furnace l1. Additional coke may also be charged to the smelting furnace H to supply the heat required for the reduction by combustion therein, but this heat or a substantial part of it is with advantage supplied electrically. In the smelting furnace H, the alumina content of the ore is dissolved in depleted electrolyte, the fluoride fusion, from the last cell of the series, the sixtieth cell in the series illustrated, and th thus regenerated electrolyte is transferred to the first filter 41 after separation from the bulk of the iron, silicon and titanium in the smelting furnace II. In the filter 41, the fusion from the smelting furnace I! is filtered through a body of carbonaceous material such as coke, the bulk of the remaining iron, silicon and titanium being removed by the carbonaceous material in this filter,

lplaced :upon the coke.

.zcipitated upon the coke.

andithe filtered fusion is transferred to the reactionfurnace 48. In the reaction furnace 48, the 'bulk of the remaining traces of iron, silicon and titanium are reduced and thusrendered insoluble in the fusion by reactionwith aluminum. The aluminum for this reaction is with advantage added to the reaction furnace 48 as scrap aluminum but it may be produced in this furnace by tsubjectinglthe fusion passing through the furnace to electrolysis in the furnace. The thus reduced insoluble material is then separated from the fu- .sionzby the second filtration through carbonaceous material in filter l The twice filtered, regenerated electrolyte is transferred, or permitted :to flow, from the filter 45 to the first cell of the series. The electrolyte is then circulated through the several cells of the series and a portion of its alumina content is electrolytically reduced to aluminum in each cell.

Proceeding in this manner, Ihave been able to produce directly in the first cell of the series metal analyzing better than 99.25% aluminum. The major impurities ordinarily encountered being more electro-positive than aluminum, the :metal produced in the'first-cell of the series is nor- :mally the least pure of the metal produced in any of the cells of the series and the average production of a battery 'of .say sixty cells is higher in purity than that of the metal produce'din the 'first cell of the series.

Referring to Figs. 2 and 3: The smelting furnace illustrated comprises a steel shell 28 electrically separated into two parts'by insulation between "the annular flanges near the upper end-of the :shell, an insulating refractory lining 21, a fire brick lining 22 in its upper part and a carbon lining'23 in its lower part. An opening 24 is'provided "in the upper end of the furnace through which it 'is charged and through which a carbon (electrode 2 may be inserted into'the charge'within the :furnace. A spout 26 is provided for pouring metal and fusion from the lower part of the furnace. A pair of tuyre 27 open into the furnac'e through trunnions just above the carbon lining. Carbon inserts 28 are provided for elec- L ltrical connection to'the carbon lining in'the lower part of the furnace. In this smelting furnace, alumina isdissolved in the fluoride fusion introduced through opening '2 1- from aluminiferous material in the presence of carbon at a temperature effective to reduce iron compounds present to metallic iron.

' In'one way of operating this smelting furnace, the furnace is almost filled with coke and a charge--32 of the aluminiferou material or ore to beprocessed for production of aluminum is By means of the tuyres air is then blown through the coke charge until a temperature ranging'from about 1000" C.'-to about ill50 C. is attained through the'tbody :of coke.

"cryolite or fluoride fusion mixture is with ad-' vantage introduced into the furnace in molten condition although it may be chargedzas a solid,

broken up and distributed over the body of ore, an'd'there fused. In passing through the ore, the "fusion dissolves the alumina and as the molten salt imoves downwardly through the coke, the bulk of'any iron compounds present in the :ore and carried into the fusion is reduced and pre- Depending-upon the 8 composition of the iron, this iroiima-yjmelt and .flow. to the bottom of thefurnace, or all or part of it may initially remain upon the coke. Thus a body 29 of molten fiuorides containin 'th alumina dissolved from the ore charged to the furnace, accumulates in the lower .part of the .furnace. Once the operation is established, ajbody 30 of molten iron or iron alloy also containing silicon or titanium or both is accumulated and maintained in the lower part of the furnace. If the. ore contains insuficient iron to provide this body,'scrap iron or the. like in the-requisite amount is also charged to the smelting furnace. The :fiuoride fusion, is further purified, particularly with respect to silicon and titanium, by contact at high temperature with this molten metal. To facilitate such purification, thexbody of .molten iron or iron alloy in the lower part of the furnace may be stirred, electromagnetically for example. The fluoride fusion can be further purified after .passagethrough the coke before being :poured from the furnace, by againblowingthe coke with air, through. the tuyres 21, to raise the temperature in the upper part of the furnaceto :a :point at which the reduced iron and any associated impurities melt and move downwardly through the coke and thence through the fluoride fusion into th molten metal in the lower part of the furnace. This, cycle of operations is repeated tomaintain the required supply'of fluoride fusion containing dissolved alumina to be poured-from thefurnace as required for electrolysis. The molten metal is poured from the furnace from time to time as it accumulates.

With some ores, impurities present along with :the iron, particularly phosphorus, form an impure iron in the furnace with a melting point low enough to Lpermitthe meltingof the iron reduced :by .the coke at .:a temperature not too much in excess of 1000 C. .to involve serious loss through vaporization of fiuorine'and fluorides in the furnace. However, with other ores, the composition of the iron may be such that :it :tends to remain on the coke except at temperatures excessively :high with respect to the fluoridefusion. In this event, the iron can be removed from the-coke by melting, by air-blowing, after the bulk of the fluoride fusion has-been poured from the furnace higher temperatures can beused and-appropriate provisions can be made to recover vaporized fluorine and fluorides, for example, as specifically '"described.

Instead of supplying theheat, or all of'the heat in the smelting furnace by combustion of coke,

as just described, this heat, or part of it, can with advantage be supplied by inserting the electrode "25 in the upper part of the furnace charge'and by passing an alternating current between this electrode and the carbon lining 23 in thelower part of the furnace. s

The cokefilledtower constituting the first filter designated 4-? in Fig. 1, may be constructed in the same manner as the smelting furnace illustratedin Figs. 2 and 3, although it may be smaller than the smelting furnace. Thetuyres may be omitted, but an electrode corresponding to that designated 25 and inserts connecting with the carbon lining in the lower part of the vessel corresponding to those designated 28 are with advantage provided to permit the maintenance of temperatures, by heat supplied electrically, above the melting point of fusion passing through the coke.

Referring .to Figs. 4 and The reaction fur nace illustrated comprises a cylindrical steel shell 50 supported to permit pouring through spout 5! by rotation on trunnions 52, an insulating refractory lining 53, a fire brick intermediate lining 54 and a carbon inner lining 55. A pair of carbon electrodes 56 are inserted through the openings 51 for electrically heating the reaction furnace. These electrodes may be hollow, as indicated, to permit the introduction through them and into the fusion undergoing treatment in the reaction furnace of a reducing gas such as methane. Port 58 is provided for charging fusion and aluminum to the reaction furnace.

In one way of operating this reaction furnace, a pool of molten fluoride fusion is maintained in the furnace chamber and fusion is added to and withdrawn from this pool at intervals. A temperature ranging from about 1060 C. to about 1150" C. is maintained in the furnace chamber by passing an alternating current through the pool of fusion as a resistor between the electrodes 56. As additions of fusion are made to the pool, corresponding additions of scrap aluminum in amount sufficient to reduce the traces of silicon, iron and titanium remaining in the fusion charged to this furnace are also made. This reaction furnace, however, can be operated on a batch by batch basis although control of the tem perature of the fusion in the furnace is facilitated by operating it as just described. In normal operations, the quantity of material separated from the fluoride fusion in the second carbonaceous filter, designated 49 in Fig. l, is small enough to permit the use of very simple apparatus for this step. For example, the fusion may at this point in my process be filtered by passing it through a porous carbon plate as it enters the cell or cells to which it is being supplied. This plate can be arranged in a simple vessel deep enough to provide a head of fusion sufficient to effect the fiow through the plate and well enough insulated to maintain the temperature of the fusion well above its melting point. Such carbon plates can be pre-fabricated in appropriate shapes and can be replaced from time to time as they become charged with the separated reduced and insoluble impurities.

The smelting operation described in connection with the apparatus illustrated in Figs. 2 and 3 can sometimes, with advantage, be carried out in apparatus of the type illustrated in Figs. 4 and 5 appropriately modified to provide tuyres for introduction of air, as through the trunnions 52.

Referring to Figs. 6 and 7: The cells, or as they are commonly called reduction pots, illustrated are, considered individually, conventional in character except that the steel shells usually arranged between the carbon lining and the surrounding refractory are omitted. The cell struc-- ture may include such conventional steel shells if desired. Each cell comprises a carbon cell chamber 35 supported by but thermally insulated from a concrete foundation 3! by means of a layer of refractories 38 and a pair of carbon anodes 39 suspended by metal supports 40 also serving to connect the anodes to the bus-bar system. These anodes are shown elevated above normal operating position in Figs. 6 and 7 to facilitate illustration of the rest of the cell structure. Petroleum coke is a conventional and satisfactory anode material. Iron inserts 4| in the bases of each of the several cell chambers through extensions 42 serve to connect the cell chambers to the bus-bar system. In the apparatus illustrated, each of the cells of the series is connected to the next cell by a trough which provides for transfer of the fused electrolyte seriatim from cell to cell through the series. As shown, the normal level of the liquid electrolyte in each cell chamber other than the first is slightly lower than that of the preceding chamber. Each cell chamber being connected to the next by a trough 43, the electrolyte thus flows from cell to cell through the series from the first cell to the last cell of the series. Each of these troughs is also supported by but thermally insulated from the concrete foundation 3'! by the refractory layer 38. The trough lining 44 in contact with the electrolyte is formed of carbon to resist the action of the electrolyte, but to break the electrical connection between adjacent cell chambers through the trough lining, the series of separators 45, originally of the same section as the trough, are inserted as spacers in the carbon lining of the trough between each pair of cells. These separators are with advantage fabricated as fused chromite-alumina blocks.

The cells illustrated in Figs. 6 and 7 are with advantage arranged so that the maximum electrolyte level is approximately at or somewhat below the level of the floor from which the cells are attended. Several advantages are thus secured. The cells can be positioned in a foundation supporting them laterally as well as from below; it is thus that the conventional steel shells can be omitted as previously stated to attain ims portant savings in installation costs and in maintenance costs. Breaking out of the electrolyte through the cell walls and over the floor is avoided and thus a serious danger to the operators is eliminated, shut down of cells through the running out of the electrolyte is avoided and the losses of electrolyte involved, directly and through contamination, are avoided. In my experience something more than 40% of the throw outs, that is the shut downs, of individual cells in a series during operation of the series results from such breaking out.

Referring to Figs. 8, 9 and 10: The transfer ladle illustrated comprises a steel shell 60, an insulating refractory lining 6|, an inner carbon lining 62 and a cover 63 electrically insulated from the shell 60 and lining 62 for example by an as bestos gasket 62a and by insulating washers 621). An eduction pipe 64 opens into the interior of the ladle through a carbon filter 65. An electrode 66 extends through the cover in registry with an electrode 61 secured in the lower part of the ladle to permit electrical heating of the contents of the ladle, for example, to maintain fusion temperatures. An opening 68 with a tight fitting closure is also provided in the cover 63. A spout 69 is arranged to open into the lower part of the'ladle through the seat member 10 and a plug ll, arranged to close the opening to the spout 69 through the seat member 10, is carried by an operating extension 12 passing through the cover 63, The spout 69 is with advantage of graphitized carbon. Trunnions T3 are provided to facilitate handling of the ladle and particularly the positioning of the spout 69, with an appropriate crane.

In one way of operating this transfer ladle, starting with the ladle empty and the plug II away from the seat 10, the spout 69 is inserted in the material to be transferred, for example in the pouring spout of the smelting furnace illustrated in Figs. 2 and 3, a Vacuum suificient to draw the material into the ladle is applied through connection 64, when the ladle is full the plug H is rammed into the seat 10, the vacuum is released, the spout 69 is then repositioned at the desired point of discharge and the plug 11- is withdrawn from the seat 10. material may also be accelerated by imposing a pressure through connection 64. A thermocouple is conveniently provided at a selected level at the upper part of the ladle to indicate to the operator that the fusion has reached this level when the ladle is being filled.

In some cases, the reaction furnace such as indicated at 48 in Fig. 1 may be omittedand the reaction of the once filtered fusion from the first filter, indicated at 41 in Fig. 1, with aluminum may be carried out in a ladle used for transferring fusion from the first carbonaceous filter to the second carbonaceous filter. For example, this reaction with aluminum can be effected in a transfer ladle such as is illustrated in Figs. 8, 9 and 10 by charging the required quantity of aluminum through the opening 68 after the ladl'e has been filled with fusion, the plug rammed into the seat on the bottom pouring spout and the vacuum has been released.

When the operation of the smelting furnaceror the reaction furnace involve the production of any substantial amount of fluoride fume,.I:.pass the fumefrom either or both of these furnaces through a body of the ore to be charged to the smelting furnace thus to recover and return to the operation, the fluoride material which might thus otherwise be lost. Fluorine inthe formof hydrofluoric acid being less costly: in the form of cryolite or aluminum fluoride, I may'supplymake up fluorides to the fusion by passing: hydrofluoric acid through abody of the ore to be ch'argedito the smeltin -furnace and may thereafter'adjust the composition; of the fusion by the. addition: of soda ash. Another important economy is thus effected;

In carrying out my invention in. the apparatu illustrated in the. accompanying drawings: I put each of the several. cells of a series'in operation in the usual way as individual cells; Then, with all of the cells of the series in:operation,.ll'establislr a fiow. of electrolyte through thev cells by charging a fluoride fusion containing ahighcon-r centration of dissolved alumina. to the first cell of'the series andibydischarging a fluoride fusion containing a low concentration of alumina from the last cell of-the series. As previously stated, concentration ofalumina in the fusion charged to the first cell of the series may; approximate 12-.%-14% and that of the fusion-dischargedafrom the last cell of the series may approximate 2%.-2.-5%"once the cycle of movement had. been established The-balance of the. fusion mayibe of any conventional composition. 'Ihealumina-content. of thefusion discharged from-.thelast: cell Off/he series is replenished by dissolvin -alumina in the fusion. Any, losses of fluorine or fluoride from the fusion are conveniently made upby appropriate; additions at thesametime; This addition of alumina tothe fusion constituting the electrolyte is effected in the smelting operation previously described- In: this furnace, the'-a1u'- mina. content of. the aiuminiferous material .used for the production of aluminum is, dissolved in the.:fiuor-ide' fusion, is in effect extracted With-the Discharge of the fluoride fusion, and the'fusioncontaining disa purification with respect particularly to iron, silicon and titanium. Th'e fluoride, fusion replenished with respect to alumina, and at: least partially purified, is passed through thefirst carbonaceous filter where it is subjected to a further purification with respect particularly to iron, silicon and titanium. This once filtered'fusion is then reacted with aluminum in the reaction furnace andthe bulk of the remaining-traces"of'sili con, iron and titanium are thus reduced-and rendered insoluble in-the fusion. This reduced and insoluble material is then removed in-the second carbonaceous filter where the fusion is subjected to a still further purification. The replenished andpurified fluoride fusion is then charged to'the first cell of the series. Acycle' of electrolyte flow in which the electrolyte is repeatedly replenished with dissolved alumina repeatedly purified by the double filtration coupled with the intermediate reaction with aluminum and in which the purified aluminathus supplied is progressively'electrolyzed to produce aluminum in a series of cellsis thus established.

Some aluminiferous 1 materials, rawor' partially processed bauxites, occur in a form or in-a state of subdivision such that the impurities, particularly compounds ofiron and" silicon, become dispersed: in an extraordinaryminute: state of-tsubdivision in caustic liquors used, as vinithe- Bayer. process, to dissolve: their alumina content and thus impose an unusual: burden upon. conven= tional practices'in connectionwiththe separation of such dispersed solid impurities prior t'o'precipit'ation of aluminum hydrate from the solution. My processis ofspecial'advant'age inapplication to such raw materials for the'production of aluminum. In its broader. aspect, and particularly in theconnection just noted; my process is. not-limited to operations in conjunction with cells operated. in: chemical series as well as in electrical series. It is, however, particularly advantageous-in the form in Which-it-has been describedin detail. Also, as-compared to acornpositeop'eration including the Bayer process, my process permits.- calcination of the ore, at the point it is taken from the earth", to eliminate substantially all combined aswell as uncombined moisture and. thus to eliminate as much-as 25% or more of the weight otherwise to be transported 'frommine toreduction plant, whereas suchcalcination would-render the are insoluble in the caustic liquor and therefore useless in the Bayer process.

I claim: 7

1. In -the electrolyticv production of aluminum from alumina'dissolved: in a fiuoride'fusion, the improvement which comprises circulating. an alumina-containing fluoride fusion consistingessentially of the fluoridesnof sodiumandaluminum throughaseriesof cells in each of which it is subjected: to; electrolysis i'esultingwin-.-adiminution* of thealuminacontentof tthe-fusion, dis.-

solving alumina in the alumina-depleted. fluoride the iron compounds present to metallic lron sep V arating 'in. said. aluminaereplenishing zones the l3 fluoride fusion with its alumina content thus replenished from the bulk of the iron, then filtering the separated fluoride fusion through a porous mass of carbon, subsequently reacting the filtered fusion with aluminum in a reducing atmosphere, filtering the reacted fusion through a porous mass of carbon, and again circulating the twice filtered fusion through said series of cells in repetitions of this cycle.

2. In the electrolytic production of aluminum from alumina dissolved in a fluoride fusion, the improvement which comprises circulating an alumina-containing fluoride fusion consisting essentially of the fluorides of sodium and aluminum through a series of cells in each of which it is subjected to electrolysis resulting in a diminution of the alumina content of the fusion, dissolving alumina in the alumina-depleted fluoride fusion discharged from the last cell of the series by bringing said alumina-depleted fusion in the presence of coke into contact with a charge of material consisting essentially of alumina but also including impurities of the group consisting of combined iron, silicon and titanium while maintaining a temperature effective to reduce a part of the iron compounds present to metallic iron in a smelting operation, separating the fluoride fusion with its alumina content thus replenished from the bulk of the metallic iron formed in the smelting operation, then filtering the separated fluoride fusion through a body of coke, reacting the filtered fusion with aluminum, filtering the reacted fusion through a porous mass of carbon, again circulating the twice filtered fusion through said series of cells in repetitions of this cycle, and transferring to the smelting operation coke from the body through which the fusion from the smelting operation is filtered.

3. In the electrolytic production of aluminum from alumina dissolved in a fluoride fusion, the improvement which comprises circulating an alumina-containing fluoride fusion consisting essentially of the fluorides of sodium and aluminum through a plurality of cells in which it is subjected to electrolysis resulting in a diminution of 'the alumina content of the fusion, dissolving alumina in the alumina-depleted fluoride fusion discharged from said cells by bringing said alumine-depleted fusion in an alumina-replenishing zone and in the presence of carbon into contact with a charge of material consisting essentially of alumina but also including impurities of the group consisting of combined iron, silicon and titanium while maintaining a temperature effective to reduce a part of the iron compounds to metallic iron, separating in said aluminareplenishing zone the fluoride fusion with its alumina content thus replenished from the bulk of the iron, then filtering the fusion with its alumina content thus replenished through a porous mass of carbon, reacting the filtered fusion with aluminum in a reaction zone to which aluminum is supplied in an amount suflicient to reduce remaining impurities, filtering the reacted fusion through a porous mass of carbon, again circulating the twice filtered fusion through said plurality of cells in repetitions of this cycle, and maintaining the ratio NaFIAlF3 in said fluoride fusion at a value approximating 1452100.

4. In the electrolytic production of aluminum from alumina dissolved in a fluoride fusion, the improvement which comprises circulating an alumina-containing fluoride fusion consisting essentially of the fluorides of sodium and aluminum through a series of cells in each of which it is subjected to electrolysis resulting in a diminution of the alumina content of the fusion, dissolving alumina in the alumina-depleted fluoride fusion discharged from the last cell of the series by bringing said alumina-depleted fusion in an alumina-replenishing zone and in the presence of carbon into contact with a charge of material consisting essentially of alumina but also including impurities of the group consisting of combined iron, silicon and titanium while maintaining a temperature effective to reduce a part of the iron compounds present in metallic iron, separating in said alumina-replenishing zone the fluoride fusion with its alumina content thus replenished from the bulk of the iron, then filtering the separated fluoride fusion through a porous mass of carbon, reacting the filtered fusion with aluminum, filtering the reacted fusion through a porous mass of carbon, and again circulating the twice filtered fusion through said series of cells in repetitions of this cycle.

ARTHUR F. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES P ATENTS Name Date Hall Apr. 2, 1889 Tucker July 12, 1921 Pacz Aug. 14, 1923 Rohden June 20, 1939 FOREIGN PATENTS Number Number 

